This invention relates in general to seals and more particularly to seals that are useful in hostile environments and to such seals combined with machine structures in which they are used.
The wheel bearings for road vehicles operate in a hostile environment in that they are exposed to considerable moisture as well as dust, dirt and grit, and this holds particularly true for light trucks and other vehicles equipped with four wheel drive, for these vehicles are expected to negotiate unpaved roads, as well as the terrain itself. The traditional seal for the wheel bearings of four wheel drive vehicles has two radial lips, that is a primary lip and a secondary lip, both of which contact a cylindrical sealing surface. The primary lip, which is on the lubricant side of the seal where it is exposed to the interior of the bearing, carries a garter spring which urges it tightly against the sealing surface to thereby establish a highly effective barrier along the sealing surface. That barrier retains the lubricant within the bearing. The secondary lip is maintained against the sealing surface by the natural bias of the elastomer from which it is molded, and it likewise establishes a barrier along the sealing surface, but that barrier is presented away from the interior of the bearing and thus serves to exclude contaminants from the bearing. Owing to the effectiveness of the primary lip, the secondary lip may starve for lubrication and run hot. This in turn causes the secondary lip to harden and loose its effectiveness as a barrier to the ingress of contaminants.
Because of the severity of the environment in which the typical wheel bearing seal of a four wheel drive vehicle operates, it is not uncommon to find the seal used in conjunction with another or supplementary seal located somewhat beyond its secondary lip. The supplementary seal, while perhaps providing an additional measure of protection, creates a pocket in which water and other contaminants may be trapped when the supplementary seal is less than totally effective, and that water causes the main seal to deteriorate. When this occurs, the bearing fails prematurely.
With or without a supplementary seal, radial lip seals of the type traditionally used to protect the wheel bearings on off-the-road vehicles have a somewhat limited lifespan which is considerably less than the bearings they are designed to protect. Often when a seal fails, the failure goes undetected, and by the time it is discovered, the bearing has deteriorated, indeed prematurely. Thus, the failure of a seal often results in replacement of not only the failed seal, but the bearing as well.
The seals used to protect the bearings in the wheels of large over-the-road trucks and trailers are likewise subjected to severe demands, both from the standpoint of endurance and contaminants. Whereas, the seals of off-the-road vehicles serve to retain grease, the seals for over-the-road trucks, at least those used on American highways, serve to contain oil. This derives from the fact that the level of oil within a bearing is easily ascertained when the bearing is fitted with a simple sight glass. Grease, on the other hand, does not lend itself to simple inspection procedures, and thus maintenance schedules must be established for wheel bearings to be lubricated, lest they will run dry. But oil is more difficult to contain than grease, and furthermore does not effectively lubricate those lips of the seals which are designed to exclude contaminants. This causes the lips to wear excessively which in turn diminishes their effectiveness as barriers to the ingress of contaminants. Clearly, grease would be the superior lubricant for the wheel bearings of large highway trucks were it not for the difficulty of determining the amount of grease remaining in the bearing.
On a much larger scale, sealed work roll bearings are gaining acceptance and replacing more conventional bearings which are protected with seals fitted to the chocks in which the bearings are housed. To facilitate the conversion, the sealed work roll bearing must fit in the space formerly occupied by a conventional bearing, yet must further contain seals and of course be capable of sustaining the same loads. The seals fit into the relatively small spaces at the ends of the bearings where they must effectively retain the lubricant and exclude contaminants. And contaminants are plentiful, for mill rolls run in an environment of grit and spray which can be quite harmful to a bearing if they enter the bearing.
The seal assembly of the present invention fits into the end of a bearing and provides highly effective barriers to the escape of grease-type lubricant as well as to the ingress of contamination. Yet its lips, two of which actually bear against a metal sealing surface, receive an adequate supply of lubrication and do not experience excessive wear. The seal furthermore does not require a highly effective secondary sealing arrangement to isolate the interior of the bearing from contaminants. Aside from the foregoing, the seal assembly is self-contained and can be installed as a unit in the bearing.